When I want my RKT to harden I don't use margarine and also when I get my shape finished I turn my oven on to the lowest setting (170*). When it hits temp, I turn it off and put the RKT in there on a cookie sheet with the oven door cracked open. I just did this today for the tires on the derby cake. It kind of dehydrates them. I then smear with RI to smooth out and hold everything together better. HTH